2020 Speedometer accuracy
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Joined: Feb 2020
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From: WV
2020 Speedometer accuracy
The speedometer on my 2020 RX350 with 20" wheels consistently tracks 2 mph faster at 70 MPH that what Google Maps shows. In other words, if my RX thinks it is traveling at 72 MPH, Google Maps displays 70 MPH. Perhaps Google Maps on a phone is not perfectly accurate. However, my wife's 2017 Honda Pilot tracks exactly with Google Maps using the same phone, so the vehicles are certainly different.
The 20" wheel/tire combination has a just slightly larger circumference than the 18" wheel/tire set, so the 20" set requires 3 fewer revolutions per mile than the 18" set. The wheel difference is very small, though, compared to the almost 3% difference between the vehicle and GPS measured speeds.
(Caveat: the vehicle's precise speed is difficult to see on the RX's antique analog speedometer. I noticed the behavior when having cruise set at 72 on a flat road, and the speedometer was tracking steadily at what appeared to be 72.)
Has anyone noticed a similar behavior?
The 20" wheel/tire combination has a just slightly larger circumference than the 18" wheel/tire set, so the 20" set requires 3 fewer revolutions per mile than the 18" set. The wheel difference is very small, though, compared to the almost 3% difference between the vehicle and GPS measured speeds.
(Caveat: the vehicle's precise speed is difficult to see on the RX's antique analog speedometer. I noticed the behavior when having cruise set at 72 on a flat road, and the speedometer was tracking steadily at what appeared to be 72.)
Has anyone noticed a similar behavior?
Rookie
Joined: Jul 2016
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From: Florida
Both Lexus I’ve owned (non-sport 2016 GS and now 2019 RX Sport) have had the same problem. My theory is that Lexus may be trying to save Lexus drivers a speeding ticket here and there by making you believe you are going faster 
Driver School Candidate
Joined: Nov 2017
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From: Tx
I think I am going to stay awaie at night concerned about a 2 mph difference at 70. Do you also worry that you might be traveling either more or less miles than your odometer shows? Has implications for warranty, you know.
Have you considered rolling resistance of different tire brands, effect of tire pressure on mph, different brands tires, though 20" will have different sidewall heights, levelness of roadway, etc. etd.
Have you considered rolling resistance of different tire brands, effect of tire pressure on mph, different brands tires, though 20" will have different sidewall heights, levelness of roadway, etc. etd.
Lexus Test Driver
Joined: Jul 2017
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From: NJ
I think I am going to stay awaie at night concerned about a 2 mph difference at 70. Do you also worry that you might be traveling either more or less miles than your odometer shows? Has implications for warranty, you know.
Have you considered rolling resistance of different tire brands, effect of tire pressure on mph, different brands tires, though 20" will have different sidewall heights, levelness of roadway, etc. etd.
Have you considered rolling resistance of different tire brands, effect of tire pressure on mph, different brands tires, though 20" will have different sidewall heights, levelness of roadway, etc. etd.
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From: WV
Here's a vocabulary lesson for you: curiosity and worry are not synonyms.
Racer
Joined: Feb 2017
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From: Ontario, Canada
Pretty well all manufacturers do this and for that very reason!
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From: WV
Probably a tech preview for the Lexus Safety System+ 3.0 release...
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From: Ohio
I also like to know the accuracy of the speedometer in my new cars. I timed both my 2015 and my 2018 when they were new, using a smartphone and the stopwatch app, using the mile markers on a freeway. Both cars were within 1 mph of what the speedometer showed, at 60 mph. My 2018 has the &^%$# Bridgestones, and at my 25K service they were rated "YELLOW", so mostly worn out. I should time it again and see what the effect of that wear is on speedo accuracy. Just for the sake of curiosity.
Driver School Candidate
Joined: Nov 2017
Posts: 37
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From: Tx
I also like to know the accuracy of the speedometer in my new cars. I timed both my 2015 and my 2018 when they were new, using a smartphone and the stopwatch app, using the mile markers on a freeway. Both cars were within 1 mph of what the speedometer showed, at 60 mph. My 2018 has the &^%$# Bridgestones, and at my 25K service they were rated "YELLOW", so mostly worn out. I should time it again and see what the effect of that wear is on speedo accuracy. Just for the sake of curiosity.
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From: Ohio
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From: WV
The 2020 includes a Road Sign Assist (RSA) feature, wherein it will read speed limit signs and display them in the MID. Sweet feature. There is even an option to have visual or audible alerts when exceeding the speed limit. Again, a great concept. The only alert increments, though, are for 1 MPH over, 3-over and 5-over. Weeeelll, that feature is essentially useless if the speedometer over-reports the speed. On my vehicle, for example, which indicates the vehicle is traveling 70 MPH when it is actually traveling at 68 MPH, the RSA alerts occur at 1 MPH under, 1 -over and 3-over. That's not useful. (IMHO, the RSA alerts should include an option for 8- or 10-over, too.) If the alerts were customizable I could adjust accordingly, but they are not.As an aside, the RSA visual alert made me laugh when I realized what it was. The "alert" causes the 1-pixel wide speed limit sign outline to turn from white to a dark red (on a black background). Unless you are staring at it, you'll never notice. I'd use the audio alert, but since on my vehicle the maximum alert differential is 3MPH, it sounds like a fire alarm inside the RX...
Racer
Joined: Nov 2017
Posts: 1,941
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From: New York
Ha! Here's a practical implication for those for whom curiosity is not enough. The 2020 includes a Road Sign Assist (RSA) feature, wherein it will read speed limit signs and display them in the MID. Sweet feature. There is even an option to have visual or audible alerts when exceeding the speed limit. Again, a great concept. The only alert increments, though, are for 1 MPH over, 3-over and 5-over. Weeeelll, that feature is essentially useless if the speedometer over-reports the speed. On my vehicle, for example, which indicates the vehicle is traveling 70 MPH when it is actually traveling at 68 MPH, the RSA alerts occur at 1 MPH under, 1 -over and 3-over. That's not useful. (IMHO, the RSA alerts should include an option for 8- or 10-over, too.) If the alerts were customizable I could adjust accordingly, but they are not.
As an aside, the RSA visual alert made me laugh when I realized what it was. The "alert" causes the 1-pixel wide speed limit sign outline to turn from white to a dark red (on a black background). Unless you are staring at it, you'll never notice. I'd use the audio alert, but since on my vehicle the maximum alert differential is 3MPH, it sounds like a fire alarm inside the RX...
The 2020 includes a Road Sign Assist (RSA) feature, wherein it will read speed limit signs and display them in the MID. Sweet feature. There is even an option to have visual or audible alerts when exceeding the speed limit. Again, a great concept. The only alert increments, though, are for 1 MPH over, 3-over and 5-over. Weeeelll, that feature is essentially useless if the speedometer over-reports the speed. On my vehicle, for example, which indicates the vehicle is traveling 70 MPH when it is actually traveling at 68 MPH, the RSA alerts occur at 1 MPH under, 1 -over and 3-over. That's not useful. (IMHO, the RSA alerts should include an option for 8- or 10-over, too.) If the alerts were customizable I could adjust accordingly, but they are not.As an aside, the RSA visual alert made me laugh when I realized what it was. The "alert" causes the 1-pixel wide speed limit sign outline to turn from white to a dark red (on a black background). Unless you are staring at it, you'll never notice. I'd use the audio alert, but since on my vehicle the maximum alert differential is 3MPH, it sounds like a fire alarm inside the RX...